Various forms of rheometer are known which depend upon the determination of torque required to rotate a member through a body of material the rheometric characteristics of which are under investigation.
In effect, the torque is a measure of the resistance to the movement of the member presented by the substance. Examples of known rheometers are found in Chapter III ‘Some Commercial Rotational Viscometers’ of the book ‘A Laboratory Handbook of Rheology’ by Van Wazer et al., Interscience 1966 which is incorporated herein by reference. Such devices are used to gauge characteristics such as, for example, viscosity, flow, homogeneity, etc. The torque required to rotate the member through the substance is related to the resistance to movement presented by the substance itself. While the book is of a considerable age, it is still the case that its disclosures are broadly representative of the prior art in the field of rheometry today. Consistent among the teachings of the known devices is the use of a rotating member. However, because the characteristic movement of the member is rotary, it is cyclical. In certain circumstances the cyclical nature of the movement of the member means that it passes through a disturbed region of the substance after the first passage of the member. Thus, unless the substance is one which is able substantially fully to recover before the next disturbance by the member at the rate at which it is rotating, the torque required to drive the member will be different from cycle to cycle.
The book referred to above discloses a modification of the Brookfield Viscometer on page 144 in which the basic hand-held Brookfield instrument is mounted on a ‘Helipath’ stand above a vessel containing a substance under investigation. The mounting for the device on the stand is motor driven so that it can lower the viscometer at the same time as it is rotated. The member describes a helical path as it progresses. By virtue of this helical progression, the member encounters only undisturbed material as it rotates. As is stated in the book, the Helipath Brookfield Viscometer is used for material with high yield values or extreme thixotropic or rheopectic effects. In short, materials unable to recover due to the passage of a blade can more accurately be assessed for rheometric characteristics by a member describing a helical path through the material. In keeping with the rest of conventional rheometer practice, the rheometric characteristic is assessed by monitoring the torque required to rotate the member at a given rate.
A later example of a rheometer is disclosed in EP-A-0798549, which is incorporated herein by reference. In the disclosed device the member is again both rotated and driven linearly into the substance. While monitoring the axial forces produced is intended as an option, it is for controlled compression of the substance or for feedback in controlling the device. While powders are mentioned, it fails to distinguish between them, and liquids and viscous solids. The rheometric assessment of a characteristic of the substance only involves monitoring the torque in accordance with conventional practice. Thus, torque remains the recognised quantity by which rheometric assessment is carried out.